Ethnic and socioeconomic trends in testicular cancer incidence in New Zealand

1. Ethnic and socioeconomic trends in testicular cancer incidence in New Zealand Diana Sarfati, Caroline Shaw, June Atkinson, James Stanley, Tony Blakely. University of Otago Wellington, New Zealand Results Discussion Background • Testicular cancer is the most common cancer diagnosed among young men. • Testicular cancer (TC) is increasing rapidly in developed countries, but drivers of these trends remain obscure. • Ethnic differences in TC incidence within and between countries are often marked. • White populations consistently have the highest rates of TC in all countries where data are available. • High socioeconomic status has also been found to be a risk factor for TC in many studies. • There is some evidence that the epidemiological patterns of TC may be different in New Zealand. • Rates of testicular cancer vary markedly between ethnic groups. • New Zealand is the only example where the non-white population have a higher incidence of TC than the ‘white’ population living in the same country. • The pattern of Pacific Island and Asian rates being less than European/Other men is consistent with findings from elsewhere. • In total there were2,028 cases of testicular cancer registered among men aged 15-44 years between 1981 and 2004. • We found an inverse association between socioeconomic status and testicular cancer rates even after adjusting for ethnicity. • Most but not all studies have found that men in high socioeconomic groups have an increased risk of testicular cancer. • There is also some evidence in the literature that the historical pattern by SE may be reversing in some countries, consistent with the NZ findings. Study Objective To test whether trends in testicular cancer incidence have varied by ethnicity and socioeconomic position in New Zealand between 1981 and 2004. • Why differing incidence rates and trends by social groups? • Diagnostic variation over time or between social groups. Unlikely to account for these findings. • Environmental exposures. These are likely to be important in TC aetiology. Considerable attention has been paid to antenatal, early life and peripubertal exposures, but no specific exposure has yet been identified to explain these patterns. • Genetic factors: while there is a genetic component to TC, it is unlikely that genetic variation alone is sufficient to account for striking patterns and trends over time for TC, • We found increasing rates of TC for all ethnic groups since 1990s. • Maori had higher rates, and Pacific and Asian men had lower rates than European/ Other men. • Rate ratios pooled over time were Maori: 1.51 (95% CI 1.31-1.74); Pacific0.40 (95% CI 0.26-0.61) and Asian 0.54 (95% CI 0.34-0.94) compared with European/Other men . Methods • Cohorts of the entire New Zealand population for 1981-86, 1986-91, 1991-96, 1996-2001, 2001-2004 were created from Census data and probabilistically linked to cancer records from the New Zealand Cancer Registry. • The NZ Cancer Registry is a population-based registry that collects data on the full population of New Zealand on all malignant tumours (except some skin cancers). • Men aged between 15-44 yr were included in the study. • Testicular cancer (ICD code C62) was identified from the Cancer Registry. • There are four main ethnic groups in NZ; 1) the indigenous people (Maori); 2) those of European origin; 3) People from the Pacific region and 4) the Asian group. • Household equivalised income was calculated and assigned to each individual using a NZ specific equivalisation index, and divided into tertiles. Key strength: this was a rare opportunity to simultaneously link ethnic and household income to testicular cancer records for an entire country to estimate trends over time. Key limitation: Not all eligible records could be linked to the census, but we adjusted for linkage bias using weights, and we are confident that residual bias is minimal. Conclusions • Maori men have considerably higher rates of testicular cancer than any other ethnic group in New Zealand. In all other countries, white men have the highest risk, although it is not clear why this is the case. • We also found an inverse association between household income and testicular cancer rates in contrast to the usual patterns seen historically. • The unusual trends and patterns identified in the New Zealand context may provide some aetiological clues for testicular cancer, but novel ideas are required to identify specific factors for future investigation. Analysis • There was little evidence of an income-related association (standardised for ethnicity) with TC during the 1980s. • Rates of TC appeared to increase in all income groups from the early 1990s onwards, with a relative increase in incidence of 32%, 46% and 51% for men in high, middle and low income groups respectively. • The pooled rate ratio comparing low to high income men was 1.23 (95% CI 1.05-1.44). • Incidence rates, rate ratios and rate differences (and 95% confidence intervals) were calculated after direct standardisation of the cohorts to the age structure of the 2001 WHO population. • For the income-related calculations, we also adjusted for ethnicity. • Statistical tests of trends were conducted for rates, rate differences and of the log transformed rate ratios. Sarfati D, Shaw C, Blakely T, Atkinson J, Stanley J. Ethnic and Socioeconomic trends in testicular cancer incidence in New Zealand. International Journal of Cancer. 2011: 128; 1683-91.